Triode electron gun with positive grid and modular cathode

ABSTRACT

An electron gun in which the space for electron extraction is physically separated from the space for electron acceleration. A non-emissive screen grid is located on the cathode and a second grid is located in the electron shadow of the screen grid. An anode is located beyond the second grid for accelerating the beam.

United States Patent Sommeria 51 Mar.21,1972

TRIODE ELECTRON GUN WITH I POSITIVE GRID AND MODULAR CATHODE Inventor: Jean Sommeria, Annecy, France Assignee: Societe Anonyme Alcatel-Heurtey, Paris,

France Filed: Apr. 23, 1969 Appl. No.: 818,735

Foreign Application Priority Data I Apr. 26, 1968 France ..149764 US. Cl. ..3l3/82 R, 313/250, 313/265 Int. Cl. ..l-10lj 29/02, HOlj 29/46 Field of Search ..313/82 [r l Il /m [56] References Cited UNITED STATES PATENTS 3,107,313 10/1963 Hechtel ..313/299 X 3,558,967 1/1971 Miriam ..3l5/3.5 2,758,234 8/1956 l-lensel 313/69 C 2,398,829 4/1946 l-laeff..... 313/82 X 3,154,711 10/1964 Beggs ..313/82 Primary Examiner-Robert Sega] Attorney-Littlepage, Quaintance, Wray & Aisenberg [5 7] ABSTRACT An electron gun in which the space for electron extraction is physically separated from the space for electron acceleration. A non-emissive screen grid is located on the cathode and a second grid is located in the electron shadow of the screen grid. An anode is located beyond the second grid for accelerating the beam.

1 Claims, 1 Drawing Figure I I/ ll I'l /Mm! PATENTEDMARZI I972 I 3,651,360

FIG. I

m fml him 1 m mm mm.

an 5 m me vi Invcnfor' by W, QMQESZ 14% I TRIODE ELECTRON GUN WITH POSITIVE GRID AND MODULAR CATIIODE The present invention relates to improvements in electron guns, commonly known in the art as Pierce electron guns.

In electron guns, the electrons are generally pulled from an emitting cathode and accelerated towards an anode in one single operation. Now, extracting electrons from the cathode and accelerating them towards an anode are two quite distinct physical processes, so that superimposing of both processes makes it difficult to obtain a straight line propagation of the electrons. In fact, if there is no strong accelerating field on the cathode, the electrons flow from the cathode in all directions. However setting up a strong accelerating field between cathode and anode leads to the use of prohibitive potential differences.

An object of the present invention is to obtain a straight line, parallel beam of high energy electrons by using relatively small electrical fields.

The invention consists essentially of a gun where the space in which the physical process of electron extraction takes place is separated from the space in which the physical process of electron acceleration takes place, by placing each process in a separate electrically defined space, the two spaces being adjacent and having between them a well defined separation.

Thus the invention includes the combination inside a triode electron gun, of a thermo-emissive cathode whose surface is made partially non-emissive by addition of a screen-grid; a grid positively biased in relation with the cathode, located near this cathode and of the same shape as that of the screengrid, and an anode biased positively in relation with the grid, the cathode, screen-grid and positively biased grid being preferably of shapes preventing the electrons emitted by the cathode from striking the biased grid.

In a practical embodiment, such an improved gun includes a cathode heated by a filament or by electronic bombardment a first screen-grid coated on the cathode; and a second grid, located a short distance from the cathode and brought to a positive voltage in relation with the cathode, both grids being of identical structure and having thus the same number of identically shaped meshes.

The number of meshes is preferably small but their dimensions are relatively large.

The meshes of both grids have the same arrangement, so that they can overlap each other, and the first grid acts as a screen for preventing electrons from striking the second grid; the meshes being so shaped that the elementary electron beams drawn from the second grid pass through the meshes of the second grid without striking the solid portions thereof.

In a particular embodiments of the gun, the first grid is in good mechanical contact with the cathode but in poor thermal contact with this latter, so that the first grid is at a lower temperature than the cathode and non-emissive.

A particular embodiment of an electron gun improved according to the invention is described hereunder, as a simple example implying no limitation, by reference to the enclosed drawing, where FIG. 1 illustrates diagrammatically the gun, in longitudinal section FIG. 2 shows a rational geometrical configuration of the grids.

As shown in FIG. 1, the gun includes an emissive cathode 1 fitted on a metal cylindric tube 2 supported on a part 3 made of insulating material. A filament 4 supported on a base 5 fitted on the insulating part 3 through a sleeve 6, also made of insulating material, is received into a metal tube 7 inserted into the tube 2. The tube 7 is fitted on the base 5 coaxial with the tube 2.

The filament 4 is supplied from an AC voltage source 8 and,

besides, receives together with the metal tube 7, from a DC voltage source 9, a voltage negative in relation with the cathode l.

A honeycomb screen-grid 10, such as the one shown on FIG. 2, for example, is located on the cathode I. This rid consists of seven hexagonal meshes. As shown in FIG. screen grid 10 is mounted directly on cathode l with good mechanical contact therewith. The thin support spaces screen grid '10 from cathode l, thermally isolating the screen grid from the cathode.

A metal ring 11, coaxial with the electron gun, the tubes 2 and 7 and the cathode l, is fitted at the end of another cylindric metal tube 12 also coaxial with the previous components. This tube 12 is fitted on the part 3 through a tube 14 made of insulating material.

Into the metal ring 11 is fitted a grid 15, with the same shape as the screen grid 10 illustrated in FIG. 2, and located in such a manner that it is exactly opposed and coincident to the said screen grid, a few millimeters in front of this latter.

The grid 15 is brought to a voltage positive in relation with the cathode l voltage through a DC voltage source 16.

A circular anode l7, centered on the axis of the gun, is located a few centimeters from the grid 15 and is brought to a positive voltage in relation with the potential of this latter through a DC voltage source 18. This anode 17 is, on the other side, connected to earth.

The electron gun of this structure works as follows The cathode l is heated by an electron bombardment pulled from the filament 4 by the potential difference present between the cathode l and the filament 4.

The electrons are pulled from the cathode 1 by the grid 15 which is at a positive voltage in relation with the cathode.

In FIG. 1 are shown three electron beams. These beams are, in fact, passing through three meshes of the grid 10 located on one diameter and the three corresponding meshes of the grid 15. The edges of the meshes of the grids l0 and 15 are so shaped that electrons are pulled from cathode and do not strike the solid parts of the meshes of the grid 15.

The shapes of the ring 11 and anode 17, as well as the voltages applied to these components, have been designed in order that the three electron beams illustrated on FIG. 1 join in one single cylindric beam parallel to the axis of the gun.

Whatlclaim is:

I. A high energy electron gun comprising a cathode, a thin mechanical, electrically-conductive support ring having one end fixed on the cathode and having a remote end extending outward from the cathode, a screen grid mounted on to the remote end of the thin support ring whereby the screen grid is in good mechanical contact with the cathode and poor thermal contact with the cathode, thereby thermally isolating the screen grid from the cathode, an extracting grid spaced from the cathode and defining therewith an electron extracting space, openings in the extracting grid aligned with openings in the screen grid, a first continuous power source having a negative terminal connected continuously to the cathode and hav ing a positive terminal connected to the extracting grid for continuously positively biasing the extracting grid with respect to the cathode, a ring anode having a central aperture for passing electrons spaced from the extracting grid, the extracting grid and the ring anode forming therebetween an accelerating space which is relatively long with respect to the extracting space, and a second power source connected to the cathode, the extracting grid and the anode for biasing the anode positive with respect to the extracting electrode whereby high energy electrons are extracted from the cathode by the extracting grid and are accelerated through the anode aperture by the anode, said screen grid and said extracting grid both being concave and both having the same honeycomb shape.

lOl028 04l0 

1. A high energy electron gun comprising a cathode, a thin mechanical, electrically conductive support ring having one end fixed on the cathode and having a remote end extending outward from the cathode, a screen grid mounted on to the remote end of the thin support ring whereby the screen grid is in good mechanical contact with the cathode and poor thermal contact with the cathode, thereby thermally isolating the screen grid from the cathode, an extracting grid spaced from the cathode and defining therewith an electron extracting space, openings in the extracting grid aligned with openings in the screen grid, a first continuous power source having a negative terminal connected continuously to the cathode and having a positive terminal connected to the extracting grid for continuously positively biasing the extracting grid with respect to the cathode, a ring anode having a central aperture for passing electrons spaced from the extracting grid, the extracting grid and the ring anode forming therebetWeen an accelerating space which is relatively long with respect to the extracting space, and a second power source connected to the cathode, the extracting grid and the anode for biasing the anode positive with respect to the extracting electrode whereby high energy electrons are extracted from the cathode by the extracting grid and are accelerated through the anode aperture by the anode, said screen grid and said extracting grid both being concave and both having the same honeycomb shape. 